This invention relates generally to an improved way to pour molten metal used in a casting operation, and more particularly to filtering out inclusions from the molten metal during the filling of a horizontal high pressure die casting (HPDC) shot sleeve.
Low process cost, close dimensional tolerances (near-net-shape) and smooth surface finishes are all desirable attributes that make HPDC a widely used process for the mass production of metal components. By way of example, manufacturers in the automobile industry use HPDC to produce near-net-shape aluminum alloy castings for engine, transmission and structural components. In a typical HPDC process, molten metal is typically transferred to a casting mold through a series of channels, tubes and ladles that make up a filling system. This transfer typically takes place in two steps: a low pressure pour to a filler tube (called a shot sleeve), and a high pressure injection where movement of a piston or plunger in the tube forces the metal from the shot sleeve and into the casting cavity.
One transport system for HPDC shot sleeve filling is known as a dosing furnace trough system. In this system, a pressure or pump furnace is coupled to the shot sleeve with an inclined trough (which is also referred to as a dosing launder). The furnace bottom fills the required charge weight of molten metal to one end of the trough such that it then flows the length of the trough and drains into the shot sleeve. Devices to improve metal stream control by reducing the turbulence relative to a free-falling stream of molten metal as it impinges on the shot sleeve wall and pooling metal therein may also be used. Nevertheless, undesirable high metal velocities—and the resulting filling turbulence—may still be present in the shot sleeve. Furthermore, long trough lengths also create undesirably high metal surface areas and oxide films. Moreover, this system does not include a mechanism to clean the molten metal at the shot sleeve, as the generated oxides or furnace inclusions that reside in the metal at the entrance to the trough will be transported to the shot sleeve cavity.
Another metal transport system for HPDC shot sleeve filling is known as a tilt-pour ladle system. Typical designs of this type of system are similar to a pour basin in traditional sand casting molds, but without a sprue feature. In this system, a ladle is tilted to pour the molten metal through an external spout to the shot sleeve. Metal damage results from the turbulence of the impinging metal stream on the shot sleeve surface and the pooled melt within the shot sleeve. One method of reducing this filling turbulence is the use of a device placed above the sleeve entrance that collects the ladle metal stream and directs it into the shot sleeve. As with the trough-based system mentioned above, control of the metal stream location as it hits the shot sleeve wall and pooling metal may be its primary benefit in reducing metal damage. Nevertheless, neither this nor the trough-based system has a mechanism to clean the metal, instead leaving the generated oxides or furnace inclusions to remain in the pooled metal that is resident in the shot sleeve cavity.
Another method of eliminating this filling turbulence is described in U.S. patent application Ser. No. 14/159,866, filed Jan. 21, 2014 and entitled A METAL POURING METHOD FOR THE DIE CASTING PROCESS, the entirety of which is owned by the Assignee of the present invention and hereby incorporated by reference herein. In this approach, a special tilt pour ladle couples to a side orifice in the shot sleeve and rotates to introduce metal into the sleeve cavity, after which the assembly rotates to drain the ladle and place the shot sleeve orifice at the top. While the bottom filling of this approach is especially useful in eliminating turbulence and metal damage, it may add complexity. For example, the additional joints used to establish the relative rotation are exposed to molten metal for each casting; this may exacerbate maintenance and related foundry down-time concerns. Additionally, clearance constraints of the die casting machine may hamper the ladle motion, making implementation more difficult.
An additional method of reducing metal damage during the shot sleeve filling event is described in U.S. patent application Ser. No. 14/613,991, filed Feb. 4, 2015 and entitled METAL POURING METHOD FOR THE DIE CASTING PROCESS, the entirety of which is owned by the Assignee of the present invention and hereby incorporated by reference herein. This approach uses a tilt-pour ladle with a 360 degree nozzle that is fitted with a molten metal filter. A full ladle is rotated to place the nozzle and filter proximate to the shot sleeve such that in its final position, the filter is at or near the shot sleeve bottom surface for filling. The flow restriction of the filter reduces the metal velocity and turbulence of the incoming stream, while the physical capture of melt inclusions provides an additional benefit. The present inventors have recognized that a major hurdle to achieving such filtering is that the placement of the filter (in addition to its removal for periodic cleaning or servicing) may add significant time and expense to the process, and as such may make the use of a filter untenable, especially when used in conjunction with large-scale HPDC production (where hundreds or even thousands of castings may be produced each day in a single casting machine). Moreover, storage space for filters and their replacements in or around the shot sleeve is limited, while filter mishandling may cause significant damage to the shot sleeve or other parts of the filling system, further reducing the efficiency of the casting operation.